1. Field of the Invention
The present invention relates to a non-contact electric power transmission circuit, particularly to a method for controlling an electric power transmission circuit.
2. Description of the Related Art
As to a conventional non-contact electric power transmission circuit, a resonant type full bridge circuit system is generally used as a high-efficiency non-contact electric power transmission circuit in a low DC input.
However, in a drive circuit of a general PWM control system, a duty ratio is set equal to or lower than 50% to provide a dead time in order to prevent switching elements from being simultaneously turned on to pass a short-circuit current. A period during which all the switching elements are turned off is generated due to the dead time. At this point, a voltage waveform at both ends of the switching element vibrates largely during the turn-off period. This is because an excitation current vibrates by floating capacitance or leakage inductance when the switching element is switched from the turn-on to the turn-off. Because the vibration of the voltage increases an unnecessary radiation noise, disadvantageously it is necessary to use a switching element having a high withstand voltage.
A phase shift control method is well known as the method for solving the problem. In the phase shift control method, when the switching element is turned off, a regenerative operation is performed to control the excitation current. That is, the rapid change in excitation current can be eliminated. For example, Japanese patent Application Laid-Open Nos. 2002-233158, 2005-348567, and 2006-197711 disclose the phase shift control method.
A general phase shift switching operation will be briefly described with reference to a block diagram of FIG. 1. When a first switching element Q1 and a fourth switching element Q4 of a full bridge circuit 2 are turned on, a positive current is passed through an electric power transmission coil L1. Then the first switching element Q1 is turned off, and energy regeneration is performed by the fourth switching element Q4 and body diode of a second switching element Q2. Then the fourth switching element Q4 and the second switching element Q2 are turned on to continue the energy regeneration, and a direction of the current is reversed to continue the energy regeneration. The fourth switching element Q4 is turned off, and the body diode of the fourth switching element Q4 and the second switching element Q2 are turned on to continue the energy regeneration. Then the second switching element Q2 and a third switching element Q3 are turned on to pass a negative current through the electric power transmission coil L1. The second switching element Q2 is turned off, and the body diode of the first switching element Q1 and the third switching element Q3 perform the energy regeneration. Then the first switching element Q1 and the third switching element Q3 are turned on to continue the energy regeneration, and the energy regeneration is continued even if the direction of the current is reversed. Then the third switching element Q3 is turned off, and the first switching element Q1 and the body diode of the third switching element Q3 perform the energy regeneration. Then the first switching element Q1 and the fourth switching element Q4 are immediately turned on to complete one-period operation. In the general phase shift switching operation, the regenerative operation is performed by a combination of the first and third switching elements Q1 and Q3 and a combination of the second and fourth switching elements Q2 and Q4.
However, in the phase shift control method, a pulse of a duty ratio that is determined in principle is shifted to perform the control. A closed-loop feedback pulse signal is always necessary to restrict an operating range from a light load to an overload. On the other hand, in a method for controlling the non-contact electric power transmission circuit, a load change is determined by detecting a current passed through an electric power transmission coil via an electric power receiving coil or a voltage generated across the power transmission coil via the electric power receiving coil. Therefore, the general phase shift control method is not suitable for the non-contact electric power transmission circuit in which the closed-loop feedback signal is hardly obtained. Because timing of the phase shift operation is difficult, a commercially available phase shift control IC is generally used. In a general specification of the phase shift control IC, an output is shut down at a maximum duty ratio from the viewpoint of an IC characteristic. When the output exceeds the maximum duty ratio, the first and second switching elements Q1 and Q2 or the third and fourth switching elements. Q3 and Q4 are simultaneously turned on to pass the short-circuit current. Therefore, generally the feedback signal is always applied, and the output is stopped when the maximum duty ratio continues.